Probiotics are back (yes, they were once a thing) and a growing number of companies claim that by adding live microorganisms, bacteria does the body good. Dannon’s Activia yogurt touts probiotics as a way to regulate the digestive system. Powerful Yogurt and Naked Pizza serve up similar health claims. The brisk business in beneficial bacteria brings in around $28 billion a year. With names like Lactobaccili, Streptococci, and Bifidobacteria, these microorganisms have been linked to lower rates of constipation, ulcerative colitis, and chronic diarrhea. Consult with Dr. Google and you might find that probiotics are the next Prozac, the cure for acne, and a surefire boost to your sex appeal.

Despite nearly a century of research—first on “scientifically soured milk” and life-prolonging yogurts—the concept remains largely under-researched and oversold. At the same time, scientists are finding that the trillions of microorganisms, as many as 10,000 different species, or about 160 species per person, flourish in our warm, wet intestines. Microbes influence our health, sometimes far beyond the gut itself. If the human microbiota, as it’s known, is an integral part of overall health and physical fitness, could microbes serve as performance-enhancing microorganisms?

One study—known, in some circles, as the “Great Balls of Fire”—claimed that probiotic yogurt imbued male mice with bigger balls and a behavior the researchers called swagger. The lead investigator, Susan Erdman, a cancer biologist at MIT in Cambridge, Mass, is a researcher with an unflagging enthusiasm for her work (and also a dedicated athlete). Three years ago, quite by chance, she told me, a colleague had noticed that the female mice in her lab colony were becoming so unbelievably shiny, they looked almost opalescent.

“I have dogs at home,” Erdman says. “Some of them go after the yogurt containers and lick them out when we’re done. I remember thinking, ‘Zappy is a black dog who glows like crazy.’” Back in the lab, colleagues began to notice that male mice had large, protruding testicles. (She showed one video of a mouse strutting around his cage like overeager stud at an oonts-oonts nightclub.) The animals exhibited slim physiques and had little abdominal fat. What gave them a youthful edge: yogurt.

Erdman suspects that bacteria confer a “glow of health” in aging animals, essentially mimicking the peak health of younger animals. In a series of recently published studies, she examined mice fed an isolated strain of bacteria called Lactobacillus reuteri, a microorganism originally isolated from human breast milk. Drinking the probiotic infusions halved a mouse’s body weight, no matter how much gooey “fast-food” chow they ate. The bacteria sped the healing of superficial skin wounds. In males, it led to heavier testes. “We ended up with a triad of features that we affectionately call shiny, skinny, and sexy,” she says. “These are indications of supreme physical fitness.”

Probiotic bacteria, which tend to number in the billions, don’t stick around with the tens of trillions of microorganisms already living in the gut. Yet, just passing through appears to shake things up and the ingested microbes stimulate the body’s immune cells, though the exact mechanism behind the effect remains unknown.

The strain Erdman studied, for example, ushered in a cascade of hormonal changes connected with an animal’s thyroid, adrenals, and gonads. “When we started feeding them microbes, the mice suddenly became more active,” she says. “How would that translate into people? You would presume that they would suddenly have more desire to get up and move around—increased energy levels, changes in metabolism, that’s mostly a good thing, right?”

So far, despite any speculation, there’s only tantalizing hints for how research in mice might translate into healthy humans. In one 2011 study in the New England Journal of Medicine, researchers examined 1.6 million years of data to determine what factors caused Americans to gain weight as we age. Potato chips, processed meats, and sugary drinks were the worst for lean body mass. People who ate yogurt, on average, lost about one pound every four years. Today, the U.S. National Institutes of Health lists hundreds of trials on probiotics—from dental health to weight loss—and most of the commercially available Lactobacilli and Bifidobacteria strains target gastrointestinal disorders.

If a particular active ingredient, or cocktails of organisms, treats diarrhea, then another set of organisms may better stave off post-marathon fatigue. (After all, you don’t grab any random pill when you’re sick.) Perhaps probiotics altering digestion also give athletes a boost by contributing to good overall health. In the few small studies thus far—hinting at longer run-to-exhaustion times and improved immune response of fatigued athletes—the existing concoctions do not appear to be a performance panacea. Moreover, because people may carry genes that make them more or less responsive to probiotics, physicians may need to get a sense of who you are as an individual before recommending a microbial cocktail that would be particularly good for bringing out the best you that you could be.

Today, there’s still a substantial gap between the lab incubator and your mouth. But looking around, you might never notice that. In the U.S., there’s no standard of labeling for probiotics. Marketing claims go largely unregulated. “If you go to your local pharmacy or supermarket, you will see shelves with compounds labelled as probiotics,” says Martin J. Blaser, a microbiologist at New York University and author of the book Missing Microbes. “Each of them has different claims for what they can do. They represent many different organisms. What I can say for sure is that almost none of them has been well tested—with respect to any of the claims.”

Not that scientists doubt the importance of these microorganisms. Far from it. Are the most important microbes may be the ones we evolved with for millions of years and acquire early in life? Blaser worries that we’re eradicating these species through the indiscriminate use of antibiotics. (Populations such as the Swedes, using far fewer antibiotics per capita, he says, are at least as healthy or healthier as we are.) Once we better know what’s missing, he says, we can expect to deliberately add them back in. “At some point in the future, we will have scientifically based, well-tested probiotics that will have specific uses to improve human health. We should be able to harness specific microbes to use them for our advantage just as we’ve harnessed microbes make our bread and brew our beer.”